循环通常依赖于一个或多个变量，你可以在循环外检查这些变量，以确保循环被正确执行。请看下例：

int x
char * cp[REQUESTED_BLOCKS]
/* Attempt (in vain, I must add... )to
     allocate 512 10KB blocks in memory.  */
for (x = 0;  x<REQUESTED_ BLOCKS ; x++ )
{
     cpi[x]= (char * ) malloc (10000,1)
     if (cp[x]= = (char * ) NULL)
           break
}
/* If x is less than REQUESTED-BLOCKS,
     the loop has ended prematurely.  */
if (x<REQUESTED_BLOCKS)
  printf ("Bummer ! My loop ended prematurely ! \n" );

注意：如果上述循环执行成功，它一定会循环512次。紧接着循环的if语句用来测试循环次数，从而判断循环是否提前结束。如果变量x的值小于512，就说明循环出错了。